Dual strength blade of 17-4ph stainless steel

ABSTRACT

A PROCESS IS DESCRIBED FOR HEAT TREATING A PRECIPITATION HARDENING STAINLESS STEEL CONTAINING, NOMINALLY, 17% CHROMIUM AND 4% NICKEL FOR OBTAINING VARIOUS MECHANICAL PROPERTIES. THE STEPS INCLUDE A SOLUTION HEAT TREATMENT, A CONDITIONING TREATMENT. A FIRST STAGE AGING TREATMENT AND A FINAL AGING TREATMENT. VARIOUS MECHANICAL PROPERTIES ARE DESCRIBED INCLUDING FATIGUE STRENGTH, AND DAMPING CAPACITY. UTILITY IS DEMONSTRATED IN THE TURBINE BLADE FIELD.

J. D. CONRAD, SR, ET AL May 1, 1913 3,730,785

DUAL STRENGTH BLADE OF l7-4PH STAINLESS STEEL Filed D60. 14, 1970 2 Sheets-Sheet l FIG. I

n m 5 m .i 2 S W IYHW T D A .IQL B 4 R F 7 E C m T 8 15m 4 3 1'2 7 0 v u 0. O O O O O O 5 M B Q N m w w n m E E M S s T 0 w w N m R D P D M u m m m "a L T. E x W P m D N S 2 a x 6 D 5 r m 0 0 0 o 0 9 B T 6 5 2 mx mmw-t.m wzrrqzmmk z m C:

NO. CYCLES OF STRESS DUAL STRENGTH BLADE OF 1'7-4PH STAINLESS STEEL Filed DOG. 14, 1970 May 1, ONRAD, 5R ET AL 2 Sheets-Sheet 2 FIG. 3

O OO2 MAX. VIBRATORY SHEAR STRESS, KSI.

United States Patent 3,730,785 DUAL STRENGTH BLADE 0F 17-4PH STAINLESS STEEL Joseph D. Conrad, Sr., Springfield, and Bruce W. Roberts,

Lester, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed Dec. 14, 1970, Ser. No. 97,555 Int. Cl. C21d 1/80;F01d 5/14 US. Cl. 148-142 7 Claims ABSTRACT OF THE DISCLOSURE A process is described for heat treating a precipitation hardening stainless steel containing, nominally, 17% chromium and 4% nickel for obtaining various mechanical properties. The steps include a solution heat treatment, a conditioning treatment, a first stage aging treatment and a final aging treatment. Various mechanical properties are described including fatigue strength, and damping capacity. Utility is demonstrated in the turbine blade fields BACKGROUND OF THE INVENTION Field of the invention Description of the prior art In US. Pats. 2,482,096, 2,482,097 and 2,482,098, a composition of matter is described which is referred to as a precipitation hardening stainless steel. Essentially, the stainless steel contains nominally about 17% chromium and about 4% nickel and various amounts of other alloying components. Most particularly, the alloy contains about 3% to about 5% of copper which through the heat treatments described in the foregoing patents functions as a precipitation hardening component to greatly enhance the mechanical properties exhibited by the steel in the fully heat treated condition.

In order to take advantage of the precipitation hardening mechanism, this prior art teaches that the steels are given a solution heat treatment in the range usually between about 1550 F. and about 2100 F. depending upon the composition employed. Such heat treatment performs the function of taking into solution into the austenitic phase all of the precipitation hardening components. Following the solution heat treatment, the material is quenched to room temperature. Depending upon the stability of the au'stenite, a transformation may take place upon such quenching or it may be necessary to subject the steel to either a double aging treatment, or a sub-zero cooling treatment in order to obtain the transformation of the austenitic phase to the martensitic phase. Where sub-zero cooling is employed, substantially all of the copper is retained in solution, whereas with a double aging treatment some of the copper is precipitated during "ice the first stage of the double aging heat treatment. In either event, the precipitation hardening reaction takes place within a completely martensitic matrix.

Following the formation of a substantially completely martensitic microstructure, the steel is then given a final aging heat treatment which aging heat treatment usually takes place at a temperature within the range between 850 and 950 F. usually for a time period of about 15 minutes and 2 hours. Thereafter the material may be quenched to room temperature. The substantially short time period for the final aging treatment is believed preferable in order to avoid the precipitation of substantially continuous grain boundary carbide precipitates which would adversely affect the corrosion resistance of the material.

While the heat treatment as above described has been effective for obtaining optimum tensile strength, yield strength, commensurate with adequate ductility properties, nonetheless such heat treatment is not effective for developing the overall optimum mechanical properties especially from the standpoint of damping capacity. The damping capacity becomes exceedingly important such, for example, as where the ultimate application of the steel material is in the form of a turbine blade. In a turbine blade, it is most desirable to have the greatest strength near the root portion of the turbine blade especially where the turbine blade root port-ion is of the fir tree or serrated root type. It is the area adjacent the root section of the blade where the greatest stresses are encountered and where the strength and ductility considerations are of primary importance. On the other hand, at the terminal portion of the turbine blade opposite the root section, where mechanical strength and ductility may be sacrificed, the important criteria are fatigue strength and damping capacity.

US. Pat. 3,210,224 addresses itself to the problem of damping capacity in precipitation harden-ing stainless steels including the composition to which the present process relates. The inventors found that by developing a cellular precipitate within the heat treated microstructure improved damping capacity was obtained.

For developing the cellular precipitate the following heat treatment was employed:

(1) Solution heat treat at 1750" F. to 2000" F. for 15 minutes to 1 hour.

(2) Quench to a temperature between F. and 350 F. and hold for 1 hour to transform the martensite obtained on quenching to a cellular precipitate.

(3) Rapidly reheating to a temperature between 1200 F. and 1700 F. for a time period between 15 minutes and 2 hours followed by air cooling to room temperature.

(4) Aging at a temperature between 1000" F. and 1200 F. for a time period between 1 hour and 10 hours followed by cooling to room temperature.

As treated, improved damping capacity was noted in the entire blade. In contrast thereto the method of the present invention is directed to the production of optimum strength and optimum damping capacity in selected places where needed, for example, in a turbine blade.

SUMMARY OF THE INVENTION The process of the present invention relates to a method of heat treating precipitation hardening stainless steels for obtaining predetermined mechanical properties at various positions including improved damping capacity. The process is applicable to a composition which includes from about 15% to about 17.5% chromium, from about 3% to about nickel, from about 3% to about 5% copper, up to about 0.13% nitrogen, up to about 0.16% carbon and the balance essentially iron with incidental impurities.

The heat treatment steps include a solution heat treatment of the entire steel member within the temperature range between about 1500 F. and about 2000" F. Thereafter the steel is quenched and following quenching, subjected to a first age hardening heat treatment. The first age hardening heat treatment takes place at a temperature within the range between about 1000" F. and about 1200 F. for a time period of about 15 minutes to about 5 hours. Thereafter, the steel member is again quenched. Following quenching the steel member is locally reheated for a predetermined portion thereof to maintain a temperature within the range between about 1100 F. and about 1400 F. for a time period of between about /2 hour and about hours, other portions of the member being below such temperature. Thereafter, the steel is cooled to room temperature where it will exhibit an optimum combination of strength and ductility properties at the end to which the solution heat treatment and single aging treatment has been applied and will exhibit optimum damping capacity where the end of the material has been subjected to the second aging heat treatment step. Between these two portions the steel will exhibit a gradient of mechanical properties depending upon which properties are selected for examination.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of the yield strength and tensile strength scatter bands versus the distance from the turbine blade tip in inches of blades which have been subjected to the method of the heat treatment of the present invention.

FIG. 2 is a plot of the fatigue strength exhibited by the material subjected to the heat treatment of the present invention.

FIG. 3 is a plot of the damping capacity of material treated according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The heat treatment of the present invention is applicable to stainless steels of the precipitation hardening variety and which have a composition which include from about to about 17.5 chromium, from about 3% to about 5% nickel, up to about 0.15% carbon and the balance essentially iron with incidental impurities. In particular, the steel should have a composition of nominally about 17% chromium, about 4% nickel, about 4% copper and up to about 0.15% of carbon plus nitrogen. Where corrosion resistance and especially stress corrosion resistance is of paramount importance, it is preferred to have nitrogen in preference to carbon however, either or both carbon and nitrogen are contemplated within the composition.

The steel is made in any well known manner and standard steelmakjng facilities are employed including the aspect of hot working and cold working to a shaped member with known intermediate heat treatments being employed.

The heat treatment of the process of the present invention may be applied to a steel member which is in its finished form, for example, a turbine blade or in the alternative, the heat treatment can be applied to the steel mill product and thereafter the finished article is formed, as by grinding, while the steel is in its fully heat treated condition. It is preferred however, to perform the heat treatment on the finished article in order to obtain optimum results.

The initial step in the heat treatment process consists of a solution heat treatment which may span a range between 1500 F. and 2000 F. The solution heat treatment preferably takes place at a temperature within the range between about 1800 F. and about 2000 F. for a time period of about 15 minutes and about 2 hours, the longer tlmes being usually at the lower temperatures.

A controlled atmosphere can be employed although it is not necessary in order to obtain the optimum results by practicing the method of the present invention. During the solution heat treatment, the alloying components are taken into solid solution within the austenitic phase of the microstructure. Usually 15 minutes suffices, however time periods of up to about 2 hours can be employed in order to insure that all of the carbon, nitrogen, nickel and copper are taken within solid solution within the austenitic microstructure.

Following solution heat treatment, it is preferred to quench the material to room temperature. While air quenching will sufiice, nonetheless oil or water can be employed. During cooling to room temperature the austenitic phase of the alloy will transform at least partlally to a martensitic phase within the microstructure. The amount of transformation is dependent to some extent upon the degree of stability of the austenite this being controlled by the actual chemical composition employed. In this respect, where the nickel, copper carbon and n trogen are on the high side, some austenite may be retained when the allow member is quenched to room temperature. However, where the aforementioned elements are on the low side of the range, the alloy may be substantially completely martensitic in structure upon quenching to room temperature.

Following quenching to room temperature, it is next preferred to subject the entire alloy member to a heat treatment at a temperature within the range between about 1000 F. and about 1200 F. Preferably for a time period ranging between about 15 minutes and about 5 hours. During this first stage aging treatment, the copper which was held in solution or taken into solution during the solution heat treatment temperature begins to precipitate throughout the microstrucure of the alloy. In addition, since the temperature is below the temperature a which martensite will retransform to austenite any remaining austenite will be unbalanced to a sufiicient degree that upon subsequent cooling to room temperature, any retained austenite will transform to martensite. It is during this first stage aging heat treatment that the copper is precipitated from the microstructure and the alloy attains its enhanced mechanical properties, namely, strength and ductility. In this respect, it is preferred to heat treat for the longer times at the lower temperature and for shorter times at the higher temperatures recited in order to obtain the desired discrete precipitation of the copper component within the microstructure to provide the enhanced mechanical properties. Once again, the alloy member may be air cooled or oil or water quenched from the first aging heat treatment temperature.

Following quenching a predetermined portion of the steel member is subjected to a second heat treatment wherein heat is applied locally to that portion of the alloy member where it is desired to obtain the improved damping capacity. In this respect a predetermined portion of the steel member is heated to a temperature within the range between about 1100 and about 1400 F. for a time period ranging between about /2 hour and about 10 hours, the shorter time periods being associated with the higher temperatures and vice versa. This heat treatment has the effect of overaging that portion of the alloy member which has been subjected to said localized heating and has the effect of vastly improving the damp- 1ng capacity of that portion of the alloy member. This becomes especially beneficial when the alloy member is in the form of a turbine blade and that portion of the turbine blade which has been given the overaging heat treatment is employed as the tip portion of the turbine blade as opposed to the root section which requires the higher strength. This differential heat treatment is accomplished by dipping the top of the blade to the desired depth in a salt bath at 1100 F. to 1400 F., the portion not immersed being cooler, with a substantially decreasing lower temperature in those portions farthest removed from the bath.

; As an alternative, to the foregoing treatment but included therein, the total heat treatment applied to a turbine blade can include a double aging heat treatment to the entire blade followed by a local aging heat treatment to a predetermined portion of the blade. In this embodiment, the same steel is given substantially the same solution heat treatment, namely, the entire member is given a solution heat treatment at a temperature within the range between about 1800 F. and about 2000 F. for a time period ranging between about /z, hour to about 2 hours. Thereafter, the member is cooled to room temperature either by air cooling or water quenching.

Following solution heat treatment the entire member is subjected to a conditioning treatment which takesplace within the range between about 1400" F. and about 1600 F. for a time period of from about 15 minutes to about 2 hours. At this temperature range copperris quite soluble in the austenitic matrix however the other austenite formers are less soluble; consequently.anyaustenite which did not transform to martensite upon cooling from the solution heat treatment, will be sufliciently unbalanced so that upon subsequent cooling from the heat conditioning treatment temperature, substantially the entire austenitic phase will have transformed to martensite.

After holding for the required timeat the required time at the required temperature, the steel is air cooled or quenched to room temperature, following which it is reheated to the aging temperature, said aging; temperature being within the range between about 1000 F. and about 1200" F. Preferably, the time at temperature will vary between about /2 hour and about 5 hours with the shorter times being associated with the higher temperatures and vice versa. After holding at the required temperature for the required period of time, the steel member is again air cooled or water quenched to room temperature. After this stage in the oveall heat treatment the copper will have precipitated from the solid solution and the entire alloy member will be in an optimum condition of strength and ductility.

Thereafter in order to get the improved damping capacity, a predetermined portion of the alloy member is subjected to an additional heat treatment at a temperature within the range between about 1100 F. and about 1400" F. for a time period between about /2 hour and about hours. Thereafter, the alloy member is cooled to room temperature and that portion of the alloy steel member in the overaged condition will exhibit improved damping capacity and yet will repossess a high degree of strength and ductility.

In order to more clearly demonstrate the process of the present invention, reference may be had to the heat treatment actually performed on a 31 inch turbine blade of the fir tree root type. The alloy having the required composition was cast into an ingot which was hot rolled to a bar and the bar was precision forged and thereafter machined to provide a turbine blade having an overall length of 31 inches from the tip of the turbine blade to the top of the fir tree root.

The following heat treatment was applied to the finished blade in the following manner. The entire blade was heat treated at a temperature of 1900 F. for a time period of 1 hour and thereafter water quenched. Next the entire blade was subjected to conditioning treatment by reheating the blade to a temperature of 1500 F. for V2 hour and followed by water quenching. Thereafter, the

entire blade was reheated to an aging temperature of 1055 F. and held at said temperature for 3 hours followed by water quenching. Thereafter, 9 inches of the tip portion of the blade was immersed in a molten salt bath maintained at a temperature of 1140 F. for a time period of 1 /2 hours followed by water quenching.

Reference is directed to the attached table which shows the results of tensile tests conducted on specimens removed from the finished blade.

TABLE Variation of mechanical properties of a 31" blade after dual tempering Elonga- Reduction Distance from UTS Y S tion, of area, tip (ins) (k.s.i.) (k.s.1) percent percent Properties same Utilizing the yield strength as the criteria, it will be notedthat the portion of the blade extending for a distance of about 9 inches from the tip exhibits somewhat lower yield strength properties. However the ductility is substantially unchanged. A gradient exists from about 10 inches to about 12 inches from the tip and the balance of 'the blade exhibits optimum strength. It should be noted that no significant differences in ductility appear over the entire span of the blade. Thus the strength is achieved in that portion of the blade where needed and thedamping capacity can be developed where needed as will be demonstrated.

Reference is now directed to FIG. 1 which graphically summarizes the data from the table. The upper scatter band identified as 10 is the ultimate tensile strength where as the lower scatter band 12 is the yield strength. It will be noted that by the method of the present invention, lower strengths have resulted from the treatment for developing damping capacity, nonetheless, the data are reproducible and clearly define the transitional area from about the 10-inch position on the blade up to about the 12-inch portion. Beyond about the 12 inch position the properties assume a relatively constant level.

Reference to FIG. 2, demonstrates that the fatigue life exhibited by the material is unaffected by the heat treatment involved in the method of the present invention. Fatigue life remains substantially constant over the entire length.

Perhaps the most significant improvement is graphically illustrated by reference to FIG. 3. In FIG. 3, the damping capacity of the material is illustrated from two different portions of the blade. The curve 20 is the damping capacity for material taken from that portion of the blade nearest the root. On the other hand, the curve 22 is the damping capacity of material taken from the tip section of the blade. The damping capacity has almost doubled by the employment of the process of the present invention. Thus it is possible to develop the strength where it is neded most and without sacrificing ductility develop the damping capacity where it is needed.

We claim as our invention:

1. In the method of heat treating a precipitation hardening stainless steel turbine blade for obtaining predetermined mechanical properties at various positions including improved damping capacity, said steel having a composition including from about 15 to about 17.5% chromium, from about 3% to about 5% nickel, from about 3% to about 5% copper, up to about. 0.13% nitrogen, up to about 0.15% carbon and the balance essentially iron with incidental impurities, the steps comprising, solution heat treating said turbine blade at a temperature within the range between about 1500" F. and about 2000 F., quenching, heating to an age hardening temperature within the range between about 1000 F. and about 1200 F., quenching, and locally reheating a predetermined portion of said turbine blade to a temperature within the range between about I100 F. and 'abouf"1'400F."for a time period of between about /2 hour and 10 hours, the shorter times being associated with the higher temperatures and vice versa, said locally reheated portion of the turbine blade exhibiting improved damping capacity.

2. The method of claim 1 in which the turbine blade is solution heat treated for a time period of between about 15 minutes and about 2 hours.

3. The method of claim 1 in which the turbine blade is given a first aging treatment for a time period between about /2 hour and about hours.

4. The method of claim 1 in which the turbine blade is air cooledto about room temperature after each heating step.

5. In the method of heat treating a stainless steel turbine blade for obtaining predetermined mechanical properties at various positions including improved damping capacity said steel having a composition including from about 15% to about 17.5% chromium, from about 3% to about 5% nickel, from about 3% to about 5% copper, up to about 0.13% nitrogen, up to about 0.15% carbon and the balance essentially iron with incidental impurities, the steps comprising, heating the turbine blade to a solution heat treatment temperature within the range between about 1800 F. and about 2000 F. for a time period of between about /2 hour and about 2 hours and thereafter cooling the same to room temperature, reheating the turbine blade to a conditioning treatment temperature within the range between about 1400 F. and about 1600 F. for a time period of from about 15 minutes to about 2 hours and thereafter cooling the same to room temperature, reheating the turbine blade to an aging temperature within the range between about 1000 F. and about 1200 F. for a time period of between about /2 hour and about 5 hours and thereafter cooling the same to room temperature, and locally reheating a predetermined portion of said turbine blade to a tem- 8 perature within-the range between about I F. and about 1400 F. for a time of between about /2 hour andabout' 10 hours and thereafter cooling the turbine blade to room temperature, said locally reheated portion of 'the'turbine blade exhibiting improveddamping capacity.

6. The method of claim 5 which includes a water quenching following each heating step.

7. A turbine blade formed of a precipitation hardenin g stainless steel having a composition consisting essentially of from about 15% to about 17.5% chromium, from about 3% to about 5% nickel, from about 3% to about 5% copper, up to about 0.13% nitrogen, up to about 0.15 carbon and the balance essentially iron with incidental impurities, said turbine blade being character ized'by a predetermined root portion and a predetermined tip portion, the root portion having optimum strength and ductility and the tip portion having optimum damping capacity and a gradient of properties exhibited by that portion of the blade between said root portion and said tip portion, said properties being imparted to the steel by practicing the method of claim 1.

References Cited UNITED STATES PATENTS 2,920,007 1/ 1960 Buckland 416241 3,312,449 4/1967 Chandley 4l6-241 3,210,224 10/1965 Argo 148142 3,316,129 4/1967 Token et a1. 148142 X 3,331,715 7/1967 Bulina et al 148-142 3,342,455 9/1967 Fleck et a1 416-241 3,355,333 11/1967 Haynes et a1. 148142 3,549,273 12/1970 Bird et al. 416-241 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 148-39; 416241 

